The invention relates to a damping circuit for an energy storage device and to a method for damping oscillations of the output current of an energy storage device, in particular for battery converter circuits for supplying voltage as a variable current source for loads occurring such as, for example, electric machines in drive systems of electrically operated watercraft or land vehicles.
It would appear that, in the future, electronic systems which combine novel energy storage technologies with electric drive technology will be used increasingly both in stationary applications, such as wind turbines or solar power installations, for example, and in vehicles such as hybrid or electric vehicles.
In order to feed alternating current into an electric machine, a DC voltage provided by a DC voltage intermediate circuit is conventionally converted into a three-phase AC voltage via a converter in the form of a pulse-controlled inverter. The DC voltage intermediate circuit is fed by a string of battery modules connected in series. In order to be able to meet the requirements placed on power and energy which are provided for a specific application, a plurality of battery modules are often connected in series in a traction battery. Such an energy storage system is often used, for example, in electrically operated vehicles.
A series circuit comprising a plurality of battery modules is associated with the problem that the entire string fails when a single battery module fails. Such a failure of the energy supply string can result in failure of the entire system. Furthermore, temporarily or permanently occurring reductions in power of a single battery module can result in reductions in power in the entire energy supply string.
Documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclose battery cells connected to one another in modular fashion in energy storage devices which can be coupled into or decoupled from the string of battery cells connected to one another in series selectively via suitable actuation of coupling units. Systems of this type are known under the name Battery Direct Converter (BDC). Such systems comprise DC sources in an energy storage module string which can be connected to a DC voltage intermediate circuit for supplying electrical energy to an electric machine or an electrical power supply system via a pulse-controlled inverter.
BDCs generally have increased efficiency and increased fail safety in comparison with conventional systems. The fail safety is ensured, inter alia, by virtue of the fact that defective, failed or incorrectly functioning battery cells can be disconnected from the energy supply strings by suitable bridging activation of the coupling units.
The energy storage module strings in this case have a plurality of energy storage modules connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit, which makes it possible to bridge the respectively associated at least one battery cell or to connect the respectively associated at least one battery cell into the respective energy storage module string depending on control signals. Optionally, the coupling unit can be designed in such a way that it additionally makes it possible to connect the respectively associated at least one battery cell into the respective energy storage module string even with inverse polarity or else to interrupt the respective energy storage module string.
The total output voltage of BDCs is determined by the actuation state of the coupling unit and can be set gradually, wherein the graduation of the total output voltage is dependent on the individual voltages of the energy storage modules. Owing to intrinsic complex impedances of the energy storage modules and their components, the energy storage device acts with a downstream intermediate circuit capacitor as a resonant circuit. The resonant frequency of this resonant circuit can vary with the number, which differs depending on the voltage requirement, and the clock-pulse rate of the connected energy storage modules. This means that, when the energy storage device is coupled to a load acting as variable current source, such as an inverter and an electric machine connected downstream of the inverter, for example, undesired resonances can occur.
There is therefore a need for measures with which the occurrence of such resonances or current fluctuations during coupling of a BDC to an intermediate circuit capacitor for feeding a load acting as variable current source can be reduced or suppressed.